It is well known in the art to use local oscillators (LOs) to drive dual conversion circuits. In such circuits, a first LO signal is mixed with an RF signal in a first mixer to generate a first intermediate frequency (IF) signal. Then, the first IF signal is mixed with a second LO signal in a second mixer to generate a second IF signal. The frequencies of the first and second LO signals are usually selected so that the first and second IF signals occur either at a specific frequency or within a specified frequency range.
The LOs may generate spurious signals due to injection locking, and at harmonic, and subharmonic frequencies of the desired signal. One problem in the prior art occurs when these spurious signals feed back into the RF input or couple to some other part of the conversion circuit. Signal coupling is likely to occur when the local oscillators and the conversion circuit are constructed on an integrated circuit substrate. One method of eliminating the effects of spurious signals is to use band pass filters which attenuate the spurious frequencies in the IF signals. A problem with this method arises when the spurious signals are close in frequency to the incoming RF signal or to the selected IF signals.
It is typical in the prior art to make the reference frequency which creates the LO output reference frequency equal to the step size of the frequency synthesizer system. It is further typical of the prior art to employ a single loop frequency synthesizer to create the first LO in tuners. For example, if the step size of the system is 62.5 Khz, then the reference frequency to the single loop PLL would also be 62.5 Khz.
It is highly desirable to suppress harmonics and spurs of the reference that are in band to a level below the noise floor of the VCO, requiring the loop bandwidth of the PLL to be less than the reference frequency. In the case where the reference is the step size, the loop bandwidth is rather narrow.
Consequently, it is desirable to provide both a small step size as well as a wide bandwidth for the LO signal providing for enhanced immunity to spurs as well as providing for a narrow bandwidth COHO in the tuner.
A television tuner that is constructed on a integrated circuit substrate is disclosed in the pending patent application entitled MONOLITHIC TELEVISION TUNER, filed Apr. 21, 1995 and assigned Ser. No. 08/426,080. However, that application does not disclose the present system and method for an interference free broadband tuner circuit.